Since the 1980's when the Global Positioning System (“GPS”) constellation of radio navigation satellites was being deployed, Government and industry have been working towards making satellite radio navigation more and more accurate and available. First envisioned as a military utility to be mainly used by aircraft and ground troops, the signal has always been thought of as weak. While strong enough to be used outdoors when there is a clear line of sight to the satellite, users have pushed for the ability to use it where signals have been weakened by going through building walls or trees.
Two areas that present some of the highest challenges to such a system include first, the ability to track indoors in challenging attenuated environments, and secondly, the ability to reduce or eliminate multipath errors; now considered one of the more dominant errors in the system.
GPS utilizes the L-Band portion of the Radio Frequency (“RF”) spectrum which inherently does not transmit well through structures, foliage, buildings, and other attenuated environments. This loss of signal can prevent a receiver from obtaining all satellites in view and in many cases cannot receive a solution or may arbitrarily lock onto a false multipath signal. This produces either a very poor solution or no solution at all. Because of poor signal reception, many receivers go into a “code” tracking mode where the “carrier” cannot be tracked and the poorer resolution code loops take over to give a deeper tracking capability. However, code loop tracking with low Signal to Noise Ratio (“SNR”) can result in large tracking errors, which result in poor accuracy solutions.
Additionally Phase lock loops, Costas loops, delay lock loops traditionally used for tracking purposes can be fairly unstable and unable to track beyond typically 15 degrees of phase tracking error. Robust solutions that can track over all these conditions have been sought. As a whole, the industry has been researching and developing solutions to address these issues for years but have been unable to achieve a high accuracy and robust solution for a practical implementation and cost.
A few of the primary methods that have been developed to address some of these issues are a) enhanced phase lock loop, Costas loop and delay lock loop variants; b) aided solutions using fusion from optical, network aiding, integration of Wi-Fi, cellular and other RF signals of opportunity, c) long coherent duration integration using high quality inertials and high quality clocks, and d) Real time kinematic techniques and differential GPS to track the carrier phase cycles for a high precision application; e) long duration non-coherent tracking methods.
All of these methods and others have generally fallen short in that they only address a portion of the problem set and in many cases are not practical implementation for the average mobile navigation user. Solutions using enhanced phase look loops, current attempts in developing fusion solutions, and long coherent track attempts employing INS and/or high quality clocks all have issues involving complexity, cost and integration challenges. While non-coherent track can extend low signal code tracking, it also has correlation integration time limitations and measurement accuracy limitations compared to the low level coherent signal tracking achievable by the current invention. In addition non-coherent methods in many cases do not address multi-path errors. Although the background problem is stated in terms of a GPS system, the techniques described herein can be applied to any time varying signal processing analysis, including but not limited to radio signals, acoustic signals, imaging signals, biological signals, RADAR signals, and communication signals.